High vacuum fuse



NOV. 7, 1933. WELLMAN ET AL 1,934,458

HIGH VACUUM FUSE Filed March 23, 1928 Fig. 2.

hm/entors I Da\/icl C. Prince Ber-tram We! Im an,

b filwwweefl Their Attorn ey.

Patented Nov. 7, 1933 UNITED STATES PATENT OFFICE HIGH VACUUM FUSE Application March 23, 1928. Serial No. 264,113

1 7 Claims.

Our invention relates to fuses for interrupting electric circuits and particularly to enclosed fuses comprising a combination. of elements which cooperate in a way to secure new results by utilizing certain principles of electronic emission in a very high vacuum.

While certain principles of our invention may be applied to the construction of fuses for direct current circuits, one of our main objects is to provide a fuse peculiarly adapted for operation on alternating current circuits and which, while also applicable to circuits of moderate voltage, is suitable for operationon commercial circuits of 13,200 volts and over.

A further object of our invention is to provide a fuse which shall interrupt an alternating current circuit only at substantially the zero point of the current wave and thus interrupt the flow of power without the production of voltage surges on the system. 4 V

A further object of our invention is to provide a fuse which will interrupt a -relatively high voltage alternating current circuit without the dissipation of a large amount of energy at the fuse, whereby the fuse will be free from the hazard of bursting or exploding andconsequent damage to property and danger to life.

A further object is to providea fuse which when it has blown leaves a gap of exceptionally high dielectric strength between the terminals whereby there is no danger from subsequent breakdown across the blown fuse.

Among the advantages of our invention which are of great commercial importance are that the fuse reduces fire hazard to a minimum; is noiseless in operation; relatively small, relatively inexpensive, and quite definite in its action of interrupting the circuit when that current value for which the fuse is designed to operate is reached.

All prior fuses with which we are familiar, whether of the open or enclosed type, or expulsion type, or oil immersed type, when used on the alternating current circuits are as likely to break the circuit at a high point on the current wave as at any other point and consequently generally cut off the flow of power at some other point than substantially at the zero point. Consequently when used on commercial circuits of even voltages of the order of 2,000 to 15,000 volts, a very large amount of power is dissipated at the fuse when it blows. The resistance of the arc which occurs when a fuse thus blows at other than zero point tends to reduce the current very rapidly, and the inductive reactance of the system produces a high voltage in an endeavor to keep the current from thus rapidly decreasing. This high voltage is a factor in the amount of energy dissipated at the fuse and furthermore the sudden surges of voltage produced tend to puncture the insulation at other points of the system as well as tend to cause the fuse to arc over across its terminals. The voltage peaks may cause the arc to reestablish itself and thus persist for several half cycles thus adding to the amount of .energy to .be dissipated before the circult is cleared if it clears at all. The explosive force exerted at the fuse is tremendous on a moderately high voltage circuit supplied from a generating station or system of present day commercial size. The fire hazard is considerable and the report or noise when the fuse goes is startling. Moreover the current value at which the fuse blows is uncertain and to a considerable extent dependent upon whether the overload which blows the fuse comes on slowly orsuddenly as in case of a shortcircuit. Fuses of the expulsion type depend upon an explosive action to open the circuit so that when the current gradually rises until the fuse blows there. may be no violent explosion and the fuse fails altogether to clear.

In accordance with our invention the fuse is maintained in a very high vacuum which not only protects the fuse metal from oxidation or other deteriorating influences, but presents such definite conditions for loss of heat by conduction and radiation that the fuse operates with considerably greater uniformity at its designed current value. Further in accordance with our invention the fuse for alternating current circuits functions to keep the current flowing by electronic emission with very little voltage across the gap until substantially the zero point of the current wave is reached at which time the emission of electrons ceases almost instantly. Relatively little energy is therefore dissipated at the fuse and voltage surges are eliminated.

Our invention will be better understood from the following description considered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing Fig. 1 represents a fuse embodying our invention; Fig. 2 indicates a modified form of fuse vessel; Fig. 3 diagrammatically represents a further modification and one ap plication of our fuse to protect an instrument transformer, and Fig. 4 represents a modified construction of fuse embodyin certain broad the ends of the terminals 3 1 frequency heating features of our invention together with certain improvements.

In Fig. 1, we have represented one example of a fuse embodying our invention. In this embodiment the vessel is indicated as made of vitreous material such as quartz or a suitable glass, examples of which are well known in the art of electron discharge devices, X-ray apparatus and the like. As here shown the vessel is provided with a bulbous central portion. The fuse link 2 is located centrally of the bulbous portion and the ends of the fuse link are secured in any suitable manner to terminals 3 and 4 which are connected by the conductors 5 and;

6 to terminal caps 7 and 8. respectively, which are indicated as extending over and secured in any suitable manner as, for example, by cement to the neck portions of the vessel. The cylindrical portions of the caps "I and 8 are adapted to be inserted in fuse holding members or clips not illustrated.

It .isimportant for alternating current work that the terminal members 3 and'4 be of a material which is adapted to emit electrons by the cathode spot effect, as hereinafter described, without producing a substantial emission of electrons'byrthe'hot cathode effect which occurs with refractorymetals such as tungsten. Low melting'point metals such as copper, silver and nickel are suitable and aluminum willalso be satisfactory. The material of. which the fuse linkiitself-is made is :not of the same importance but tungsten is-a satisfactory metal and has the advantage that'it can be raised to a very high temperature by passing heating current therethrough while the vessel is being evacuated so that it can be quite completely freed of occluded gases. a

Where the fuse is to carry moderate values of current the leading in conductors 5 and 6 may be sealed to the stems 9 and 10 of the vessel by a pinch seal; Examples of suitable leading in conductors adapted to make a tight and permanent seal with glass are now well known in the-art of incandescent lamps and high 'vacuum devices. Such leading. in wires are described for example in Letters Patent to Fink No. 1,498,908 dated June 24, 1924. The ends of the stems 9 and 10 are provided with shields 11 and 12 to protect the seals against of electron bombardment during the operation the injurious effect of the fuse in interrupting a circuit as hereinafter described. For alternating current work the shields 11 and 12 if located relatively near and 4 should also be made of metal which is not adapted to reach such an extremely high temperature as to produce a substantial electron emission by the hot cathode efiect. Nickel is an example of a suitable metal for these shields. Shields of metal such as molybdenum are frequently used in high voltage high vacuum electron discharge devices for the reason that such a metal may be readily raised to a sufficiently high temperature by high to drive out the occluded gases during exhaust, but since the shields are made of reasonably thin metal they can be satisfactorily heated even though they are of a lower v melting point metal such as nickel.

The very best technique for securing an extremely high vacuum is important for securing the best results when the fuse operates. The vessel should be baked during exhaust to drive out occluded gases from the walls and all surfaces and metal parts within the vessel should be freed from occluded gases. It is preferable to de-gas metal parts such as the terminal members 3 and 4 and also the shields l1 and 12 before they are assembled in the vessel. As pointed out in Letters Patent to Langmuir No. 1,558,436, dated October 20, 1925, metal which has been freed from occluded gas does not readily reabsorb gas even though it is exposed to the air or other gases. This makes it possible thoroughly to de-gas such parts as desired before assembling in the vessel and facilitates the final operation of de-gasing and exhausting after assembly.

After the fuse elements have been assembled in the vessel sufficient current should be passed through the fuse to raise it to a temperature sufficient to drive out occluded gases while the exhausting operation is going on. The walls of the vessel should be thoroughly baked out and suitable means utilized, for example high frequency heating, to assist in raising the metal parts in the vessel to a sufficiently high temperature to insure that after the vessel has been sealed off an extremely high vacuum will continue to be maintained although the fuse may become very hot when carrying currents approximating the value at which the fuse is designed to interrupt the circuit.

In-the drawing the projection 13 indicates one suitable place at which the vessel may be sealed off after the desired high vacuum has been produced. As indicated in the drawing this location of the projection enables it to be covered by the metal fuse caps and protected from injury during handling of the fuse.

Experience'with fuses of the high vacuum type described will enable one to determine from the appearance of a fuse when a predetermined test ing current is passed through it whether or not the vacuum is of the high order desired before the fuse is put into service on a line. If desired, however, any well known method of testing such a fuse may be employed to determine whether or not'the vacuum is of the desired order.

In order better to understand the new functions and results of our invention we will now describe the action which we believe explains why the fuse will interrupt a high voltage circuit only at substantially the zero point of the current wave. When an excess of current, due for example to an'overload, occurs the fuse link 2 is broken. Almost instantly the entire fuse link is consumed or disintegrated but current continues to be carried through the vessel by reason of an electron discharge between the terminals 3 and 4 until the current wave to its zero value. To understand this phenomena it should be borne in mind that there are two types of electron emission. Thermionic emission is produced by the heating of a metal to the point where the electrons in it have sufficient energy to breakthrough the metal boundary into space. This emission phenomena is described by the Richardson law. The second kind of electron emission which we utilize in our alternating current fuse we will call cathode spot emission. If

we assume that at the time the fuse blows and the link is vaporized the terminal 3 is acting as the cathode or negative terminal for the particular half wave of alternating current then obtaining there will be a point of the soft metal terminal 3 to which the fuse link was attached which will be vaporized by the current flowing. We believe the action to be explainable on the assumption that "the breaking of the fuse link produces such a potential gradient at the cathode 1 falls substantially 3 that electrons continue to be pulled out of the soft me'talterminal at a temperature much lower thanthat corresponding to emission from ahot cathode of refractory-metal such as tungsten.

At the time the metallic circuit is broken and the unvaporized metal terminals are still close together the potential drop acrosswhatever gap may exist at that time'is millions of volts per centimeter even though the voltage drop across the fuse caps niay'be very low'. That is a small drop'i'n potential applied across'an extremely small-space=gives a high potential' gradient. A gradient 'ofth'e order of millions'of volts per centimeter will draw electrons from a relatively cold metal, that is, a metal too cold to emit an appreciable number of electrons in accordance with "Richardson's law; These electrons 'will form collisions with the vaporized metal in the space. These-'collisions will'ionize some of the metal vapor thusproducin'gadditional electrons which join those"already-"produced and travel toward theanode o'r' positive-electrode which correspondsto'the terminal 4in the example we are considering. The positive ions travel toward thecathode or negatively charged electrode. Due to theirmuch greater mass the motion of the positive ions while absolutelyvery rapid is relati'vely slow-compared"to'that'of the electrons so that positive chargesaccumulate in the region adjacent to the cathode and these'f positive charges maintaina high potential gradient at the" cathode surface thus continuing to pull' out 'elect rorijs' ev'en though the anode may be at considerable distance and have become the terminal 4 iii} the" 'ca'se' 'of 'the' fus'e we 'are' "considering; The pesiti'veions continue to move towards the cathode and as theystrike' it they maintain heat at the cathodesurface" ata point where further metal is' vaporizd thus maintainin'ga supply of metal vap'or'to be ionized-by collision from-the electrons. The process-once initiated can therefore be'ni'aintained-between the'spaced terminals 3 and 4, Inthe region not immediately adjacent to the'cathode therewill be no highpotential gradient as such regions will be filled with a substantially equal number of electrons and positive ions so that by far the greatest part of the'potential drop occurs immediately adjacent the cathode;- Thi'scathode spot effect is so"effective that a voltage of approximately volts is' -en tirely' 'ade'quate -to maintain a large current between the electrodes and'at the same time maintain' a gradient at the'cathode surface of the order of a million volts per centimeter. When the current carried by the cathode spot comes to zero, as it doesat the termination of the half cycle of the'current' wave, the cathode spot on this-electrode will be extinguished and the conditions-required for restarting will no longer exist. At this time the voltage between the electrodes, namely, terminals 3 and 4, will reverse so that the electrode 4 which in the first half cycle was considered positive will now be negative. However, the potential gradient required for the formation of a cathode spot does not exist at the surface of this electrode so that no cathode spot is formed. Aconsiderable fraction of a half cycle is available during which the cathode spot on the other electrode 3 now positive can be extinguished and all of the attendant ionization and metal vapor dissipated. The cathode spot extinction occurs in a time of about two micro-seconds which means that the fuse should still be operative tointerrupt the circuit if the frequency were raised to a point where a half cycle occupies about two micro-seconds' that is a frequency of about 250,000 cycles 'per'second Owing to the extremely short period of time required for extinction of the cathode spotit is apparcntthat on a circuit having a frequency of cycles the current wave will have progressed through only about three minutes of arc during the time required forthe cathode spot to be extinguished.

A cathode spot is easily maintained for an indefiniteperiod with a voltage of the order of 50 volts and currents'as'low as 50 or 60 amperes where softmetal electrodes such as copper 'or nickel are used where one of the electrodes remains thecathode or negative'electrode. Since large currents may be carried by the electron discharge'and carried-at such a 'low voltage drop, the amount -of energy which must be dissipated in the fuse is relatively small. Most of the voltage dlOpTiIl a'short circuit, for example, takes place throughout other portionsof the circuit instead of-across the fuse as in the case of fuses of the priorart with which we are familiar. On a comine'rcialcircuit-of 13,200 volts," for example, it is apparent that the 50 volt drop across the fuse is very small and for very high voltages insignificantycompared to the voltage drop in the rest of thecirc'uit and it is not"'material in proportion to the voltage on the restofthe' circuit in a comj'rnercial circuit-of 2300'volts. Moreover since the electron discharge is so easily maintained after having once-been started; and is extinguished so quickly when the current reverses, it is apparent that with our fuse the-current continues to flow inthe circuit substantially to the end of the half cycle in which the fuse'blows and that the flow of power is-cut off only at substantially the zero point o'f the 'eurrent wave. 'The circuit is thus broken at the'id'eal moment and voltage surges which wouldtend to flash across the terminals outside the fuse, or puncture the insulation at some other onthe circuit in which the fuse is connected, are eliminated,

Moreover since the amount of energy that must b 'diss ipatedafter'the fuse link'is broken is so smau'the usual tremendous explosive effects are absent and a relatively small 'vessel is adequate for a fuse of considerable voltage-and capacity.

When the fuse blowsfit merely glows brightly for a moment. Its operation is entirelywithout'noise since there is no air in the vessel to transmit sound waves. I v

In determining the size of the fuse vessel there are two requirements which must be may First the size of the vessel should be large enough so that 'nopart of the fuse link before or during blowing can come into contact with the glass walls. If such contact should occur it may irn pair the vacuum without breaking the envelope because the hot metal will cause the glass to evolve gas. Or the wall may be cracked by the hot wire or the wall. may be punctured due to high potential gradients in the glass due to the wire point against it. Inany event the vacuum is impaired and its dielectric strength lost.

The distance between the fuse terminals 3 and 4 can be made relatively small so that a relatively short fuse link can be used. This reduces the amount of gas and metal vapor which may be evolved when the fuse blows and simplifies the problem of preventing contact between t e fuse and the wall of the vessel. A gap between electrodes of the order of of an inch in air will break down at approximately 30,000 volts. As the pressure "is reduoed b'e'low atmospheric pressure the voltage required tobreak down the gap is at first decreased until it reaches a minimum of from 100 to 200 volts at a pressure of a few microns (millionths of a meter). As the pressure is reduced still further the voltage necessary to break downthe gap steadily rises'until values of the order of millions of volts per .centimeterpotential gradient arerequired in approximately perfect vacuum It .is thus necessary for a vacuum fuse to have a vacuum which is more perfect, than a small fraction of a micron for any commercial voltage and the excellence of the fuse improves as the vacuum is made better and better. A vacuum which will remain of the orderof .01 or .001 of a micron is satisfactory but as above indicated an extremely high vacuum is important and should remain high after the fuse has blown.

Tests have shown that fuses may be built to interrupt several hundred amperes at conmiercial voltages such as 13,200 volts without making the fuse vessel of large sizeand yet maintain such a high vacuum that the gap may not be subsequently broken down with testing potentials of the order of 100,000 volts. There is undoubtedly a volume below which it is unadvisable. to go for the vessel of a successful fuse for a given capacity and voltage on accountof the evolution of small amounts of gas when .the fuse link blows. These amounts of gas are too small to raise the pressure enough to cause failure in vessels of rather small size; Usingv a glass vessel with a. bulb five inches in diameter and=a-fuselink of the order, of 1 inches in length we have successfully interrupted over 400 amperes at 13,200 volts. The fuse vesselcould probably be made still smaller for such service, but the example given conveys a fair ideaof the size of the vessel required for lower voltages. We believe it to be entirely possible to build fuses for the highest commercial voltages which are at present about 220,000 volts. The size of the vessel for such extremely high voltage service, with the vessel mountedin air, would be determined principally by the distance by which the metal terminals outside ofthe vessel must be separated toprevent the fuse from arcing over on the outside. By immersing the entire vessel and its outside terminals in oil, the size of the vessel required for adequate spacing of such terminals may be reduced.

For fuses of large capacity and moderate voltage it may be desirable to use a metal vessel instead of a glass vessel. The technique of making proper seals with metal vessels is now known to those skilled in the art. Examples of-such seals are shown in Letters Patent to Kruhet al. No. 1,564,690, dated December 8, 1925, and to Housekeeper No. 1,294,466, dated February 18, 1919. For the extremely high voltage circuits there will not be the same advantage in using a metal vessel because of-the fact that the size of the device is limited by the creeping and striking distance of the circuit voltage along the outside and a metal vessel being a conductor will be disadvantageous in this respect.

Suitableremotely controlled or manually operated disconnecting switches may be provided for opening the circuit at points each side of the fuse after it has blown so that a new fuse may be inserted. The operation of the fuse is so successful and it so much less expensive than an adequate oil switch that we anticipate that our high vacuum fuses will have a considerable field of use as a substitute for such switches. Automatically operating mechanism may be provided to substitute another fuse in the circuit when one fuse has blown. Thefuse may also be used as a safety device in series with an oil switch which normally controls the circuit, the fuse operating whenever the switch fails to open the circuit.

Referring to Fig. 2 a modified form of fuse vessel is indicated, the; vessel being cylindrical throughout. The diameter of the vessel will be made adequate and the principles which govern thiswill be obvious to those skilled in the art from the foregoing discussion. The terminal caps have not beenillustratedin this figurebut it will-be understood that an arrangement like that of Fig. lis suitable.

;In Fig. 3 we illustrate a modification of-our invention wherein the fuse link is kept taut by a suitable spring 14; We have found, however, that such a spring is not necessary and that it is not necessary that-thefuse be kept taut. The spring where used should be made of refractory metal so that-it can be readily freed of occluded gases. The geometry of the tube and the mounting should-also be such that there is no danger of the end of the spring orthe copper or other soft met-a1 terminal 15 mounted thereon from touching the walls of the vessel. As indicated in the drawing,-the spring is preferably more or less covered .by the terminal 15 to prevent the refractory spring from acting as a hot cathode with respect to the anode.

Fig. 3 also illustrates an application of our fuse for which we regard it as well suited, namely, for the protection of potential transformers such as instrument transformers. In Fig. 3 the primary of the potential transformer is indicated at 16 and its secondary at 17. The secondary suppliesa suitable measuring instrument indicated as a voltmeter 18. The fuse may be made quite small for this work by reason of the fact that a current limiting resistor 19 may be used in series with the fuse. The primary of the transformer is indicated as connected across the line conductorsv 20 and 21 which represent a source the voltage of which is to be indicated by the voltmeter.

In Fig. 4 a high vacuum fuse embodying the broad principles of our invention is disclosed together with a further improvement which facilitates the process of freeing the fuse supporting terminals from occluded gases. As shown in this figure the fuse supporting terminals 22 and 23 are made in the form of hairpin like loops. For alternating current service these loops are made of low melting point metal such as copper or pure nickel and the final operation of depriving the loop terminals of occluded gas is facilitated by the fact that a local heating current may be passed through each of the loops by connecting the leading in wires for the respective loops to suitable sources of current. Currents can thus be utilized which are as large as necessary without dependence upon the amount of current the fuse link itself can carry. Loops of considerable size can thereby be readily brought to the temperature necessary to free them from occluded gases While the final exhausting operation is taking place and the necessary current can also be passed from a suitable source of current through the fuse link so as to bring it also to the desired high temperature. The arrangement shown in Fig. 4 is disclosed and claimed in the copending application of David C. Prince, Serial No. 264,117 for High vacuum fuse filed concurrent-1y herewith and assigned to the same assignee as the present application.

The principles of our invention may also be applied to fuses for direct current circuits but for that relation of utility it is important that the fuse terminals or at least that terminal which is the cathode shall be of a high melting point metal such as tungsten which is not adapted to emit electrons at relatively low temperature by the cathode spot effect. The cathode should also be constructed so that heat may be conducted away from it readily and it should therefore preferably be of reasonably large size unless special cooling means are provided. If when the fuse blows some point of the tungsten cathode is heated sufficiently hot to emit electrons thermionically or by the hot cathode effect the current will continue to flow but will tend to reduce in value as the gap across which the discharge is taking place increases. It is diliicult to secure sufficient metal vapor from the tungsten cathode to maintain a cathode spot. Several hundred amperes at 250 volts have been tried and found insufficient where tungsten contacts are separated in a high vacuum. With direct current flowing, the cathode will continuously be cooled by the emission of electrons and by conduction through and radiation from the metal until the temperature is too low to emit further electrons thermionically after which the circuit will be opened. The small amount of vapor produced from the tungsten cathode is not sufficient to result in a positive ion bombardment of the cathode suflicient to restore as much heat as is conducted away from a properly designed cathode. The thermionic electron emission of course bombards the anode while the current is flowing and raises the anode to a high temperature but when the circuit is interrupted the anode cools down. Such tungsten fuse terminals are not well adapted for operation to interrupt alter nating current because of the fact that the electron bombardment from the cathode to the tungsten anode raises the anode to such a high temperature that it is in a condition to emit electrons thermionically to restart the discharge when the alternating current wave reverses so as to make the fuse terminal which formerly was the anode now the cathode. The heating of the anodes from electron bombardment is so effective that the current flow may be maintained in an alternating current circuit after the fuse has blown by reason of the alternating heating of the electrodes and the hot cathode electron emission occurring alternately from one fuse terminal and then the other.

While we have illustrated certain embodiments of our invention and described its theory of operation as we understand it and described certain relations of utility for our invention it is to be understood that we are not bound by any theory of operation, and it is moreover apparent that modifications and changes will occur to those skilled in the art without departing from our invention, and it is contemplated that our fuse may find a greater field of use in some application other than we have described and we therefore aim in the appended claims to cover all modifications and applications within the true spirit and scope of our invention.

What we claim as new and desireto secure by Letters Patent of the United States, is:

1. A fuse for interrupting analternating current circuit including a highly evacuated vessel, spaced terminals therein and a fuse link supported between said terminals, the fuse link and terminals and the inner walls of said vessel being thorough- 1y freed of occluded gas, said terminals being composed of a metal adapted to emit electrons at a relatively low temperature with a voltage drop of the order of 50 volts across the fuse terminals when the fuse is blown, said vessel being evacuated to provide a vacuum of the order of one-thousandth of a micron.

v 2. A high vacuum alternating current fuse including a vessel, a pair of spaced terminals within the vessel and a fuse link supported between the said terminals, the terminals being composed of a metal adapted to emit electrons by the type of emission which is characteristic of copper and nickel, said fuse and terminals and the inner Walls of said vessel being thoroughly freed of occluded gas and the vessel being evacuated to provide a vacuum as high as practicable and better than one-tenth of a micron.

3. A high vacuum fuse including an evacuated vessel, the vacuum within said vessel being as high as it is practicable to attain and better than one one-hundredth of a micron, a pair of terminals composed of a metal adapted to emit electrons by the type of emission which is characteristic of copper and nickel extending within 1 said vessel and spaced a comparatively short distance apart, and a fusible element of small mass located between and interconnecting said terminals, the inner walls of said vessel and all of the elements contained therein being thoroughly 1' freed of occluded gas.

4. A high vacuum alternating current fuse including in combination a vessel evacuated to an extremely high degree of vacuum, a fusible element and means for supporting said element 1" within the vessel, said supporting means being constituted of a metal which in a high vacuum will not support electron emission thermionically. 5. A fuse for protecting an alternating current circuit comprising a highly evacuated vessel and l a fusible element located therein, and a pair of spaced terminals for supporting said fusible element, said spaced terminals being composed of a metal which is adapted to produce a character of electron emission which will be extinguished in 1 a time of the order of a few micro-seconds whereby in the event of the blowing of the fuse the current will be maintained until substantially the end of the half cycle and Without restarting in the next half cycle. 1.

6. An alternating current fuse including an evacuated vessel, a pair of spaced terminals and a fusible element between said terminals within said vessel, said terminals consisting of a metal which will support an electron discharge at low l voltage between said terminals when the fuse blows but without attaining a temperature sufiicient for substantial emission of electrons thermionically when acting as a cathode, the elements within said vessel and the walls of said vessel 1 being thoroughly freed from occluded gas and the vacuum being extremely high.

7. A vacuum fuse for high tension alternating current circuits comprising an evacuated vessel,

a pair of spaced terminals of material capable i i;

of cathode spot electron emission mounted within the vessel, and a fusible element disposed between said terminals, all of the elements within the vessel and the inner walls of the vessel being thoroughly freed from occluded gas, the vacuum within the container being better than one onehundredth of a micron.

BERTRAM WELLMAN. DAVID C. PRINCE. 

